Is a qualified person’s failure to step forward to lead the prayer—in a mosque where there is no regular Imam—considered humility on their part, or is there a concern that it might be a forfeiture of good, especially if the one who does step forward to lead is not better than them in memorization or recitation?

The Sunnah is for the one who is most knowledgeable of Allah's Book to lead the people in prayer. Therefore, it is incumbent upon the most knowledgeable person to step forward for the Imamate, in compliance with the Prophet's command (peace be upon him), and not to wait for people to insist. However, if his stepping forward for the Imamate leads to a harm, such as alienating the hearts of some of the congregation, then it is better to step back. And abandoning some recommended acts to unite hearts is a legitimate practice.